The present invention relates generally to pressure sensing devices, and more particularly to a pressure sensor device and methods employing surface-launched acoustic wave devices, a method of use thereof, and a manufacturing method therefor. Additionally, the present invention relates to wirelessly interrogable pressure sensing devices.
In recent years, surface-launched acoustic wave sensors, and specifically surface acoustic wave (SAW) sensors, have gained significant recognition as tools for measuring physical and chemical parameters in a wide variety of applications. SAW sensor technology offers the following general advantages: passive device operation (no battery); potential for use as a sensor and an RF transmitter when queried, providing wireless operation; small size, low cost, rugged construction, and ease of production in high volume using standard process equipment. Some of the physical parameters measured using SAW sensors include temperature, pressure, strain, acceleration, and torque.
Numerous potential applications have been mentioned in the literature for such sensors, one of which is the measurement of the pressure and/or temperature within tires. Tire pressure sensors using SAW devices have been demonstrated in the technical literature. Temperature sensors have also been demonstrated using SAW devices. Additionally, at least one patent has been issued relating to the use of SAW devices as sensors for tire pressure (U.S. Pat. No. 6,003,378 to Siemens).
However, each of the approaches to tire pressure sensing using SAW devices known in the art is lacking in some way. Primarily, these approaches are difficult to manufacture and package in high volume, and thus would be prohibitively expensive for the desired application. Further, the known devices are believed to be too large to be practicable over a long time within a tire, which is an environment subject to extremes of temperature and forces. Additionally, many of the known approaches are not temperature compensated, which results in inaccuracies in measurements due to variations in temperature. A brief description of two of the most relevant prior art examples is necessary to demonstrate the advantages of the present invention.
One approach to sensing strain using SAW devices has been to fabricate SAW devices on both sides of a piezoelectric substrate that is supported at one end, as in a cantilevered beam. Bending of this beam then results in one side undergoing compression while the other side undergoes expansion. The compressed side experiences an increase in SAW velocity and thus frequency (or a reduction in delay), while the expanded side experiences a decrease in SAW velocity and a corresponding reduction in SAW frequency (or an increase in delay)
The double-sided approach has the distinct advantage of being inherently temperature compensated, since any fluctuation in temperature would have a uniform effect on the piece of piezoelectric substrate, and would result in the same change in SAW velocity due to temperature in both devices. This concept can easily be extended to a multiply supported membrane, with one side sealed to a reference chamber. In this configuration, the device becomes a pressure sensor. The reference chamber can be filled with a desired pressure (from vacuum to a desired set pressure), and as long as this reference chamber is hermetically sealed to the piezoelectric substrate, it provides a stable reference pressure for comparison. The piezoelectric membrane would then deform in response to changes in external pressure, causing complementary changes in frequency or delay of the SAW devices on either side of the membrane.
Practical difficulties arise, however, when considering how to manufacture and package such devices. Fabricating SAW device on two sides of a wafer, while possible, is considerably more complicated than the standard single-sided fabrication processes known and used in the art. Additionally, the surface of the SAW device must be protected from contamination, while still being exposed to the pressure to be measured, because contaminants that adsorb onto the surface would cause changes in the SAW performance that would confound the measurements being made. Protecting the surface of the SAW without reducing the sensitivity of the device is a difficult problem, and one for which an adequate solution has not (to Applicants"" knowledge) been found. Finally, making electrical connections to this double-sided device and connecting the device to an antenna for wireless interrogation are quite challenging tasks.
U.S. Pat. No. 6,003,378 to Scherr et al. teaches a wirelessly interrogable pressure sensor using SAW elements wherein a reflective delay line with at least three reflectors is positioned on a pressure sensing membrane such that it extends over both an expanding and a compressing region of the membrane. When subjected to a change in pressure, the reflectors located in regions of compression and expansion undergo shifts in acoustic wave velocity and hence in the phase angle of the reflected signal. Such shifts in phase angle can be measured and provide information on the pressure change that has occurred. Plate bending is used in this device, requiring a much larger device than would otherwise be needed in order to achieve the desired complementary stress distributions within the substrate. In this and other publications based on this work, the device is described as having the piezoelectric substrate of the SAW device as the pressure sensing membrane, packaged using an AQP (all-quartz package) approach. This involves joining two quartz plates together using some mechanical support and adhesive joint material such as a glass frit seal. This process is not compatible with conventional SAW device packaging and manufacturing techniques, and results in a device that, while responsive to pressure, is relatively fragile, is expensive to produce, and cannot be used to measure temperature.
Based on the foregoing descriptions, the main drawbacks to the known prior art technology are:
1. Devices are not manufacturable in high volume.
2. Final devices are not as rugged or robust as necessary for applications in harsh environments.
3. Device sizes are larger than desired.
4. Devices are more expensive to produce than desired, requiring nonstandard manufacturing techniques.
5. Some devices are not temperature compensated.
6. Double-sided sensor devices would be difficult to package without causing a reduction in sensitivity.
7. Known devices that can provide both pressure and temperature measurement are impracticably and impractically large.
It is therefore an object of the present invention to provide a wirelessly interrogable SAW-based sensor.
It is a further object to provide such a sensor that is capable of meeting the rigorous challenges encountered within such environments as the interior of a tire and during tire manufacturing process.
It is another object to provide such a sensor that is capable of producing a sensitive, temperature-compensated pressure measurement.
It is an additional object to provide such a sensor that provides temperature information in addition to the pressure measurement.
It is also an object to provide such a sensor that is manufacturable using known, relatively standard techniques.
It is yet a further object to provide such a sensor that can be produced in high volumes and at low cost.
It is yet another object to provide a sensing system using such a sensor.
It is yet an additional object to provide a method for using such a sensor.
A further object is to provide a method for manufacturing such a sensor.
These and other objects, features, and advantages of the invention are provided by the present invention, a pressure and temperature sensor that comprises a substantially hermetically sealed insulating package and an elastic, piezoelectric substrate deformably supported within the package along two lines substantially perpendicular to a long axis of the substrate. At least three surface-acoustic-wave resonators are affixed to a bottom of the substrate. The three resonators comprise a first and a second resonator, positioned in at least partially staggered, parallel relation along the substrate. The staggering is to permit each of the first and the second resonator to experience a different frequency shift upon the substrate""s experiencing a deformation. The parallelism is for achieving a common reference point for the deformation.
A third resonator has a long axis nonparallel to the long axes of the first and the second resonator. The temperature coefficients of the first and second resonators are substantially equivalent; that of the third is different from those of the first and the second resonator. This difference is for permitting a temperature change to be sensed and transmitted.
The sensor further comprises at least two electrical connectors. Each resonator has two electrical contacts. One contact of each resonator is connected to the first or xe2x80x9chotxe2x80x9d electrical connector, and the second contact of each resonator is connected to the second or ground connector; so the three resonators are electrically connected in parallel.
A sensing system comprises the sensor as described above and an antenna for receiving an electromagnetic signal from the three resonators.
A method of sensing pressure and temperature within a harsh environment comprises the steps of positioning a sensor such as described above within a harsh environment. An output electromagnetic signal is sent to the sensor from a location remote from the environment. The signal should have a frequency resonant with the at least three resonators and be receivable by the connector second ends. An input electromagnetic signal is received at the remote location from the sensor. The input signal is indicative of the pressure and the temperature within the environment.
A method of providing an internal tire pressure and temperature readout to a vehicle occupant comprises the steps of positioning a sensor such as described above within a vehicle tire. An input electromagnetic signal from the sensor is received at the vehicle that is indicative of the pressure and the temperature within the environment. The input signal is translated into a pressure and a temperature value, and a readout of the pressure and the temperature values are displayed within the vehicle.
A method of making a pressure and temperature sensor comprises the steps of providing an insulating package having a well therein and deformably supporting an elastic substrate within the package well. A first and a second surface-acoustic-wave resonator are affixed to a bottom of the substrate so as to have long axes parallel therebetween and to be in staggered relation along the substrate. Each of the first and the second resonators has substantially equivalent temperature coefficients.
A third surface-acoustic-wave resonator is affixed to the substrate bottom with a long axis nonparallel to the long axes of the first and the second resonator. The third resonator has a temperature coefficient different from the temperature coefficient of the first and the second resonators.
The first ends of a first, a second, and a third electrical connector are affixed in electrical contact with each of the respective first, second, and third resonators, with a second end extending out of the package.
Finally, the package is substantially hermetically sealed.